In recent years, elimination of solvents and use of water-based constitution have been demanded in the field of recording materials and an inking material utilized for printers, printing machines, markers and writing tools. Especially, there have been widely used recording materials comprising mainly an aqueous solution of a water-soluble dye and those comprising mainly a fine particle dispersion of a pigment for a water-based ink of ink-jet recording.
In a water-based ink comprising a water-soluble dye, there is used an aqueous solution of a water-soluble dye selected from the group of dyes that are categorized mainly into acid dyes, direct dyes and some food dyes. To the aqueous solution is added a glycol or an alkanolamine as a wetting agent, a surfactant or an alcohol to adjust the surface tension, thickeners and other agents according to need. The water-based inks using water-soluble dyes are most commonly used because of their high reliability of anti-clogging at pen tips or in a printer. The water-based inks using water-soluble dyes, however, easily cause blotting on recording paper because they comprise an aqueous solution of dyes. Therefore, they have only limited uses and their print quality tends to be deteriorated due to blotting of the ink. In addition, water-soluble dyes, which only permeate into a recording paper sheet and are then dried to adhere to the recording paper sheet, are hardly considered to be “dyed”. Thus, light fastness of such water-based inks is extremely low.
Further, in order to solve problems of low water resistance and low light fastness of the water-based inks utilizing the water soluble dyes, proposals of coloring water-dispersible resins with an oil soluble dye or a hydrophobic dye have been made, for example, in JP-A No. 55-139471 (the term, JP-A refers to an unexamined and published Japanese Patent Application), JP-A No. 58-45272, JP-A No. 3-250069, JP-A No. 8-253720, JP-A No. 8-92513, JP-A No. 8-183920 and JP-A No. 2001-11347.
Proposals have been made of not only dyeing a water dispersible resin with an oil soluble dye or a hydrophobic dye but also utilizing colored particles comprising a colorant and a resin that is coated with the colorant, and also utilizing colored particles comprised of a colorant and a resin the surface of which are further coated with a film-forming resin.
In many cases when a dispersion of first particles comprising a water dispersible resin dyed with these oil soluble dyes or hydrophobic dyes, second particles in which fine particles of a colorant such as a dye or a pigment are mixed with a resin, or third particles in which the first or second particles are further covered with another resin is prepared, a stable particle dispersion containing the colorant in a high concentration cannot be obtained due to insufficient solubility or affinity of the colorant or the resin to an organic solvent. The colorant dissolved or dispersed in a solvent is likely to precipitate, the resin is difficult to be mixed with the colorant, or the colorant is localized on the particle surface (the colorant is not covered with the resin entirely), and therefore, ink for ink jet printing cannot be obtained which provides an image with high density. Further, in such particles, there are problems in that the effect to improve light fastness is decreased, and properties such as dispersion stability and ink jetting stability, which are needed for ink utilized for ink-jet printing.
Some methods have been proposed, in which a colored particle dispersion formed by dispersing a mixture of a colorant and resin in water is used as an ink for ink-jet printing.
Among such the methods, examples of ones characterized in the resin used therein include that disclosed in Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP-A No. 2001-98194 in which a copolymer resin constituted by a hydrophilic polymer chain moiety and a hydrophobic polymer chain moiety is used, that disclosed in JP-A No. 2000-191968 in which a vinyl polymer formable unsaturated acid monomer, a hydroxide group-including monomer and a styrene macromer are used and that disclosed in JP-A No. 9-157508 in which a polyester resin containing a cyclohexene dicarboxylic acid is used.
However, a satisfactory colored particle cannot obtained yet since the foregoing colored particles are insufficient in their properties such as that the particle diameter thereof is as large as about 100 nm, the storage ability and the ejecting ability of the ink are low, the printing density of the ink is low and the color appearance and the light fastness of the ink are insufficient.
JP-A No. 9-157508 describes an aqueous dispersion of complex colored particle having a core/shell structure which is obtained by a procedure in which a polyester resin and an oil-soluble dye are dissolved and dispersed, and further polymerized with an ethylenic unsaturated monomer.
However, such the core/shell structured colored particle is not suitable for practical use since the particle diameter is large and there is a problem on the stability of the dispersion.
JP-A Nos. 2002-47440, 2002-88294 and 2002-97395 disclose a small diameter colored particle having a diameter of not more than 50 nm. However, the effect of the small particle is insufficient since the dispersion stability and the storage ability of the particle are insufficient and the particles are secondarily coagulated on the occasion of printing. Moreover, these publications do not describe with respect to the core/shell type particle. JP-A Nos. 2002-80746 and 2002-80772 disclose a composition in which an oil-soluble dye dissolved in a high-boiling organic solvent is dispersed in an aqueous medium. However, this composition is instable and the ejecting ability of the ink is unstable when a medium such as gelatin is not contained.
On the other hand, disclosed as methods for preparing colored minute particles is a method (e.g., JP-A No. 9-157508) in which colored minute particles are prepared by dissolving colorants in self-emulsification type polyester resins, and further monomers are added to undergo polymerization reaction. However, problems occur in which the proportion of the aforesaid colorants is reduced due to low compatibility of the aforesaid self-emulsification polymers with colorants. Further, proposed are methods in which colored minute particles are prepared by emulsifying monomers and colorants employing reactive emulsifiers so as to be radically polymerized. Examples include a method (disclosed, for example, in JP-A No. 9-279073) in which colored minute particles are prepared by emulsifying and polymerizing monomers and colorants employing reactive emulsifiers in an aqueous medium, a method (disclosed, for example, in JP-A No. 10-176130) in which colored minute particles are prepared by dissolving hydrophobic dyes in monomers and emulsifying and polymerizing the resulting composition employing reactive emulsifiers, an ink (disclosed, for example, in JP-A No. 10-251567) comprising colored minute particles which is prepared in such a manner that oil-soluble dyes are dissolved in monomers, and the resulting composition undergoes emulsification and polymerization, employing reactive emulsifiers, and further colored minute particles (disclosed, for example, in JP-A No. 2001-139607) which is prepared in such a manner that oil-soluble dyes are dissolved in monomers and after emulsifying and polymerizing the resulting composition employing reactive emulsifiers, monomers having a hydroxyl group are polymerized on the particle surface. However, in all the methods, colorants occasionally function as a polymerization trap agent. As a result, problems occur in which polymerization reaction does not proceed sufficiently. Due to that, the dispersion stability of the resulting colored minute particles is not sufficient. As a result, since problems occur in which the storage stability as well as the ejection stability of the ink is degraded, solutions are urgently sought.